Here's my solution from a while back :
For simplicity, let's suppose that there are 100 kids at the school
If 8% bring both, then 8 bring both
Let the number who bring raincoats = x
Let the number who bring umbrellas = 2x
So....the ones that bring only raincoats = x - 8
And the ones that bring only umbrellas = 2x - 8
So......we have this equation
Kids who bring only umbrellas + kids who bring both + kids who only bring raincoats = 100
(2x -8) + 8 + (x - 8) = 100 simplify
3x - 8 = 100 add 8 to both sides
3x = 108 divide both sides by 3
x = 36
So....the ones who bring only a rain coat = 36 - 8 = 28
And the ones who bring only an umbrella = 2(36) - 8 = 72 - 8 = 64 = 64%
Check
Only umbrellas + Both + Only Raincoats = 100 ???
64 + 8 + 28 = 100 (Check)